Mobile communication device

ABSTRACT

A mobile communications device has a multifrequency band antenna with a low band portion (LB) tuned to a low frequency band, and a first high band portion (HB 1 ) tuned to a first high frequency band at higher frequencies than the low frequency band. The low band portion (LB) and the first high band portion (HB 1 ) have a common first grounding point (GP 1 ), a common feeding point (FP) for feeding input signals to the antenna and for receiving signals from the antenna, and a first conductor portion (CP 1 ), which forms part of the low band portion (LB) and of the first high band portion (HB 1 ). The first conductor portion (CP 1 ) is electrically connected to the first grounding point (GP 1 ) and to the common feeding point (FP). A second high band portion (HB 2 ) is coupled to the first conductor portion (CP 1 ) and tuned to a second high frequency band at a higher frequency than the low frequency band and different from the first high frequency band. A switching network is connected between the second high band portion and ground, allowing the resonant frequency of the second high band portion to be varied, on the basis of a signal which depends on the operating mode of the device, thereby allowing four band operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/278,751, filed Mar. 27, 2001, which is hereby incorporated herein byreference in its entirety. This application also claims priority under35 U.S.C. §§ 119 and/or 365 to British Patent Application No. 0107239.6,filed on Mar. 22, 2001; the entire content of which is herebyincorporated by reference.

FIELD OF INVENTION

The invention relates to mobile communications devices such as mobiletelephones, and in particular to antennas for such devices. Portablecommunications devices are required to be compact in size, which is arequirement that applies to every component of the devices, includingthe antenna. Modern mobile telephones use two or more distinct frequencybands, and it is preferable to use the same antenna in all frequencybands used by the telephone.

BACKGROUND OF THE INVENTION

Currently, many mobile telephones use one or more of the following threefrequency bands: the GSM band centred on the frequency 900 MHz, the DSCband centred on 1800 MHz, and the PCS band centred on 1900 MHz. The 900MHz and 1800 MHz frequency bands are separated by one octave, whereasthe 1800 MHz and 1900 MHz frequency bands are separated by only afraction of one octave. In many mobile telephones using the 900 MHz and1800 MHz frequency bands, the antenna has separate portions tuned torespective ones of the two frequency bands, since it is not consideredfeasible to have one and the same portion of the antenna tuned to afrequency band of more than one octave, with a relatively large unusedfrequency band between the useful frequency bands.

U.S. Pat. No. 5,512,910 describes a microstrip antenna device havingthree resonance frequencies. However, an antenna of this type is toolarge to be used conveniently in a small mobile phone.

A known dual band antenna, as shown in U.S. Pat. No. 6,166,694, has aconductor portion, from which two spirals branch off. The two spiralsare tuned to form a high band portion and a low band portion.

European Patent Application No. 00610112.5 (not published, and notforming part of the state of the art) describes an antenna of this type,housing a second conductor, which is capacitively coupled to the firstconductor, and tuned to a second high frequency band.

It is the object of the invention to provide an antenna, which is usablein at least three frequency bands and which has the smallest possibleloss, that is the maximum possible gain, in all frequency bands.

SUMMARY OF THE INVENTION

The invention provides an antenna for use in portable communicationsdevices such as mobile telephones. The antenna is useful in a lowfrequency band and two high frequency bands, where the two highfrequency bands are relatively closer to each other than to the lowfrequency band.

The antenna includes a first radiating element and a second radiatingelement. The first radiating element has two branches, which are tunedto a high frequency band and a low frequency band. The second radiatingelement is capacitively connected to the first radiating element, andhas a tunable reactance loading, allowing the element to be tuned to asecond high frequency band, which is separate from, but close to, thefirst high frequency band. The antenna is thus effectively a triple bandantenna, and a mobile telephone having such an antenna is thus useful inthree frequency bands. For example, a mobile telephone may be made inaccordance with the invention, such that it is usable in the threefrequency bands centred on 900 MHz, 1800 MHz and 1900 MHz respectively.However, the invention is not restricted to the use in theabove-identified frequency bands, but will be suitable for use inexisting and future frequency bands as well.

It should be emphasised that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a preferred embodiment of a triple bandantenna of the invention electrically connected to a printed circuitboard.

FIG. 2 is an end view of the antenna and printed circuit board of FIG.1.

FIG. 3 schematically shows the printed circuit board with the antenna inFIG. 1.

FIG. 4 is an electrical circuit diagram showing the tunable reactanceloading of the antenna of the invention.

FIG. 5 shows an alternative form of the tunable reactance loading of theantenna of the invention.

FIG. 6 is a diagram showing a typical return loss for an antennaaccording to the invention, in a first mode.

FIG. 7 shows a typical return loss for the antenna, in a second mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The antenna according to the invention is described with reference toits use in a mobile phone. However, the invention is generallyapplicable to portable radio communication equipment or mobile radioterminals, such as mobile telephones, pagers, communicators, electronicorganisers, smartphones, personal digital assistants (PDAs), or thelike.

FIGS. 1-3 show a printed circuit board PCB with an antenna 10 accordingto the invention, suitable for use in a mobile telephone. In theillustrated embodiment, the printed circuit board has a rectangularshape, but of course the invention is not restricted to the use of arectangular shape. In practice, the printed circuit board will have anumber of electronic components mounted thereon, which are necessary forthe operation of the mobile telephone, but which are not part of theinvention. In FIG. 3 such components are therefore indicated onlyschematically.

In FIGS. 1-3 an electrically conductive material, such as copper,constitutes the antenna 10 of the invention. The antenna is preferablyspaced from the printed circuit board PCB with a predetermined distancetherebetween. A first conductor portion CP1, which is rectilinear inthis embodiment, has a ground point with a first grounding post GP1 at afirst end of the first conductor portion CP1. In use, the groundingpoint will be electrically connected through the first grounding postGP1 to ground potential on the printed circuit board PCB. Near the firstend, at a predefined distance therefrom, the first conductor portion CP1has a feeding point with a feeding post FP electrically connecting thefirst conductor portion CP1 to an electronic circuit on the PCB forfeeding the antenna with signals to be transmitted by the antenna,and/or to electronic circuitry for receiving signals received by theantenna.

The portion of the first conductor portion CP1 situated between thefeeding post FP and the first grounding post GP1 functions as a matchingbridge MB.

At a second end, opposite the first end, a low band portion LB branchesoff at one side of the straight first conductor portion CP1 and forms aspiral. Specifically, three rectilinear segments LBa, LBb, LBc, formingright angles with each other, constitute the low band spiral. Theinnermost segment LBc in the spiral is wider than the remaining threerectilinear segments including the first conductor portion CP1.

Between the first and second ends of the first conductor portion CP1, afirst high band portion HB1, also forming a spiral, branches off at aright angle to the same side as the low band portion LB. The first highband spiral HB1 is also constituted by three rectilinear segments HB1 a,HB1 b, HB1 c, forming right angles with each other. The segmentsconstituting the first high band spiral could have substantially equalwidths, the third segment HB1 c could be wider than HB1 a or HB1 b asshown in FIG. 1, or other relative widths could be chosen.

The low band portion LB of the antenna is tuned to have a relatively lowresonance frequency, such as 900 MHz, and a predefined bandwidth todefine a low frequency band of the antenna. The low resonance frequencyis mainly determined or influenced by the length of the low band portionLB measured from the feeding point FP to the inner end of the spiral,which length corresponds to one quarter of a wavelength at the lowresonance frequency. When an electrical signal with frequencies in thelow frequency band is fed to the feeding point FP of the antenna,corresponding electromagnetic signals will be radiated from the low bandportion LB of the antenna as radio waves; and, vice versa, when theantenna receives electromagnetic signals in the form of radio waves withfrequencies in the low frequency band, electrical signals will begenerated by the low band portion LB of the antenna, and the thusgenerated electrical signals are sensed at the feeding post FP byreceiving electronic circuitry connected to the antenna.

The first high band portion HB1 of the antenna is tuned to have a firsthigh resonance frequency, and predefined bandwidth to define a firsthigh frequency band. The first high resonance frequency is mainlydetermined or influenced by the length of the first high band portionHB1 measured from the feeding point FP to the inner end of the spiral,which length corresponds to one quarter of a wavelength at the firsthigh resonance frequency. When an electrical signal with frequencies inthe first high frequency band is fed to the feeding point FP of theantenna, corresponding electromagnetic signals will be radiated from thefirst high band portion HB1 of the antenna as radio waves, and, viceversa, when the antenna receives electromagnetic signals in the form ofradio waves with frequencies in the first high frequency band,electrical signals will be generated by the first high band portion HB1of the antenna, and the thus generated electrical signals are alsosensed at the feeding point FP by receiving electronic circuitryconnected to the antenna.

In accordance with the invention the antenna also has a second high bandportion HB2 in the form of a second conductor portion CP2 arranged in aparallel relationship to the first conductor portion CP1 and at apredetermined distance therefrom. The first and second conductorportions are each typically 1.5-2.0 mm wide. At a first end, the secondhigh band portion HB2 has a grounding point, which is electricallyconnected to a second grounding post GP2. The second grounding post GP2is arranged close to feeding post FP, preferably at a distance of 0.5mm, or at least in the range between 0.1 mm and 1.0 mm.

Together the first conductor portion CP1 and the second conductorportion CP2 form an electrical capacitor. A capacitive or parasiticcoupling therefore exists between the first conductor portion CP1 andthe second conductor portion CP2.

Further, the device includes a switching network SN, which is connectedbetween the second grounding post GP2 and ground potential on the PCB.Thus, the grounding point of the second high band portion HB2 iselectrically connected through the second grounding post GP2 and via theswitching network SN to ground potential on the PCB.

FIG. 4 shows the arrangement of the switching network SN, including aninput 40 for connection to the second grounding post GP2. The input 40is connected to ground through a reactive element, in this example aninductor L.

A capacitor C and a PIN diode D are connected in parallel with theinductor L. A serial link consisting of a further inductor Lbias and aresistor Rbias is connected to the anode of the diode D, and fed with abias voltage VDC. A further capacitor Cbias is connected between thebias voltage VDC and ground.

Thus, depending on the value of the bias voltage VDC, the reactanceconnected between the input 40 and ground will vary. The diode Doperates as a switch such that, when a specific value of the biasvoltage VDC is applied, the inductor L is shorted out of the circuit,thereby altering the reactance of the switching network SN which isconnected between the input 40 and ground.

Other switching networks, for example using varactor diodes or a MicroElectroMechanical System (MEMS) can be used to provide a variablereactance in a somewhat similar way.

FIG. 5 shows the use of a switching network SN based on a MicroElectroMechanical System. Specifically, FIG. 5 shows the switchingnetwork SN including a MEMS switching network 42, and a variablereactance element 44. The switching network SN has an input 40 forconnection to the second grounding post GP2, which is then connected toground through the switching network SN. The variable reactance element44 includes at least one reactance element, such as a capacitor 51,inductor 52, and short-circuit 53, connected in series with respectiveswitches 54, 55, 56 of the MEMS device 42. Other elements can beprovided as required to produce the necessary reactance values. Theswitches are then operated by a control signal 57, so that the reactiveelements are switched into and out of the circuit path, therebyproviding different reactance values between the grounding post GP2 andground.

Thus, the resonance frequency of the second high band resonator HB2 ismainly determined or influenced by: the length of the second conductorportion CP2, which approximately corresponds to one quarter of awavelength at the second high frequency; the gap between the firstconductor portion CP1 and the second conductor portion CP2, and hencethe capacitive coupling between them; and the value of the variablereactance connected between the input 40 and ground.

Advantageously, the second high band portion HB2 can be tuned to aresonant frequency close to that of the first high band portion HB1. Thetwo resonant frequencies of the first high band portion HB1 and secondhigh band portion HB2 can be in separate bands or can form one broadband.

In a preferred embodiment of the invention, the bias voltage VDC cantake two values, a first of which tunes the second high band portion HB2of the antenna to a resonance at a second high resonance frequency closeto the first high resonance frequency, while the second value tunes thesecond high band portion HB2 of the antenna to a resonance at a thirdhigh resonance frequency, which is also close to the first highresonance frequency, but different from the second high resonancefrequency.

The second and third high resonance frequencies can be chosen to behigher or lower than the first high resonance frequency, as desired.

When an electrical signal with frequencies in the frequency band of thesecond high band portion HB2 is fed to the feeding post FP of theantenna, these signals will be coupled to the second conductor portionCP2, due to the tuning of the capacitive or parasitic coupling existingbetween the first conductor portion CP1 and the second conductor portionCP2, and corresponding electromagnetic signals will be radiated from thesecond high band portion HB2 of the antenna as radio waves. When theantenna receives electromagnetic signals in the form of radio waves withfrequencies in the frequency band of the second high band portion HB2,electrical signals will, conversely, be generated by the second highband portion HB2 of the antenna, and these signals will be coupled tothe first conductor portion CP1, and the thus generated electricalsignals are also sensed at the feeding post FP by receiving electroniccircuitry connected to the antenna.

FIG. 6 shows a typical return loss for a multi frequency band antennaaccording to the invention, in a first mode of operation, when theswitch is on (that is, VDC is high), the inductor L is shorted out ofthe circuit by the capacitor C and diode D. The return loss is heredrawn on a linear frequency scale from 500 MHz to 2.5 GHz.

It can be seen that the return loss has one distinct minimum at a lowfrequency band, namely at about 900 MHz, and two minima at two highfrequency bands HF2, which are relatively close to each other, namelythe PCS band at about 1.9 GHz and the UMTS band at about 2.2 GHz.

FIG. 7 shows the typical return loss for the multi frequency bandantenna according to the invention, in a second mode of operation, whenthe switch is off, that is VDC is low (at or close to 0 V), and theinductor L is in the signal path.

In this case, it can be seen that the return loss again has one distinctminimum at a low frequency band, namely at about 900 MHz, because thelow resonance frequency is unaffected by the switching, and two minimaat two high frequency bands, which again are relatively close to eachother, namely the PCS band at about 1.9 GHz and the DCS band at about1.8 GHz.

The bias voltage VDC can therefore be provided by a control circuit ofthe phone which controls the mode of operation thereof, thereby ensuringthat the antenna is in the first operating mode when UMTS operation isrequired, and is in the second operating mode when DCS operation isrequired.

It will be noted in FIGS. 1 and 3 that the first high band portion HB1of the antenna is arranged on one side of the first linear conductorportion CP1, and the second high band portion HB2 of the antenna isarranged on the opposite side of first linear conductor portion CP1.This has the effect that interference between the two high frequencybands is reduced to a minimum.

In FIG. 3 it is seen most clearly that the active portions of theantenna (including the linear conductor portions CP1 and CP2, and thelow and the spiral conductor portions LB, HB1) are spaced from theprinted circuit board PCB. In the space between the active portions ofthe antenna and the PCB there is a dielectric substrate DE with physicaldimensions and specific dielectric properties selected for the properfunctioning of the antenna. The thickness of the dielectric substrate DEis not necessarily the same as the distance separating the activeportions of the antenna from the printed circuit board PCB.

In each case, the bandwidth of the resonance will depend on the size andshape of the respective conductor portion, the thickness of thedielectric material, the dielectric constant of the dielectric material,the size of the antenna patch area, and the distance between the antennapatch and the edge of the PCB.

The conductor portions can be formed by punching from metal plate, or byetching. Although the conductor portions are shown as essentially twodimensional, they can be any two or three dimensional shape.

When used in a mobile telephone, the active portions of the antenna maybe placed close to the inner side of a housing wall of the telephone oreven fixed or secured thereto, for example by gluing. In that case thedielectric properties of the housing material and their influence on thefunctioning of the antenna should be taken into account.

There is thus described an antenna arrangement which can be used in afour-band phone.

1. A multi frequency band antenna comprising: a low band portion tunedto a low frequency band; and a first high band portion tuned to a firsthigh frequency band at higher frequencies than the low frequency band;wherein the low band portion and the first high band portion have: acommon first grounding point; a common feeding point for feeding inputsignals to the antenna and for outputting signals from the antenna; anda first conductor portion forming part of the low band portion and ofthe first high band portion, the first conductor portion beingelectrically connected to the first grounding point and to the commonfeeding point, the antenna further comprising: a second high bandportion, coupled to the first conductor portion and to a variablereactance, such that the second high band portion can be selectivelytuned either to a second high frequency band or to a third highfrequency band, each of the second and third high frequency bands beingat a higher frequency than the low frequency band and different from thefirst high frequency band.
 2. An antenna according to claim 1, whereinthe second high band portion includes a second conductor portioncapacitively coupled to the first conductor portion.
 3. An antennaaccording to claim 1, wherein the first conductor portion and the secondconductor portion each include substantially linear portions.
 4. Anantenna according to claim 3, wherein the second conductor portion isarranged substantially parallel to the first conductor portion.
 5. Anantenna according to claim 4, wherein the second conductor portion isarranged substantially parallel to the first conductor portion over alength approximately corresponding to one quarter of a wavelength of afrequency in the second high frequency band.
 6. An antenna according toclaim 1, wherein each of the low band portion and the first high bandportion is configured substantially in a spiral form and each branchesoff from the first conductor portion at a first side thereof.
 7. Anantenna according to claim 6, wherein the second high band portion isarranged at a second side of the first conductor portion opposite thefirst side.
 8. An antenna according to claim 1, wherein each of the lowband portion and the first high band portion includes spirals formed ofsubstantially linear portions of conductive material.
 9. An antennaaccording to claim 8, wherein successive pairs of substantially linearportions of conductive material are arranged substantially at rightangles.
 10. An antenna according to claim 1, wherein the antenna issupported on a carrier with predetermined dielectric properties.
 11. Anantenna according to claim 1, wherein the second high band portion has asecond grounding point arranged close to the feeding point of theantenna.
 12. An antenna according to claim 1, wherein the variablereactance comprises means for switching at least one reactance elementinto or out of the path between the second high band portion and ground.13. An antenna according to claim 12, comprising means for receiving acontrol signal, the reactance element being switched into or out of thepath between the second high band portion and ground, in dependence onthe control signal.
 14. An antenna according to claim 1, wherein thevariable reactance comprises a Micro ElectroMechanical System device,which is connected to receive a control signal, the value of thereactance in the path between the second high band portion and grounddepending on the control signal.
 15. A mobile communications devicehaving an antenna according claim
 1. 16. A mobile communications deviceas claimed in claim 15, comprising means for detecting a desiredcommunications mode, and selectively tuning the second high band portionto the second high frequency band or the third high frequency band independence thereon.